A number of processes for preparing formaldehyde from methanol are known (see, for example, Ullmann""s Encyclopaedia of Industrial Chemistry). The processes carried out industrially are predominantly the oxidation
CH3OH+xc2xdO2xe2x86x92CH2O+H2O
over catalysts comprising iron oxide and molybdenum oxide at from 300xc2x0 C. to 450xc2x0 C. (Formox process) and the oxidative dehydrogenation (silver catalyst process) according to:
CH3OHxe2x86x92CH2O+H2H2+xc2xdO2xe2x86x92H2O
at from 600xc2x0 C. to 720xc2x0 C. In both processes, the formaldehyde is first obtained as an aqueous solution. Particularly when used for the preparation of formaldehyde polymers and oligomers, the resulting formaldehyde has to be subjected to costly dewatering. A further disadvantage is the formation of corrosive formic acid, which has an adverse effect on the polymerization, as by-product.
The dehydrogenation of methanol enables these disadvantages to be avoided and enables, in contrast to the abovementioned processes, virtually water-free formaldehyde to be obtained directly: 
In order to achieve an ecologically and economically interesting industrial process for the dehydrogenation of methanol, the following prerequisites have to be met: The strongly endothermic reaction has to be carried out at high temperatures so as to be able to achieve high conversions. Competing secondary reactions have to be suppressed in order to achieve satisfactory selectivity to formaldehyde (without catalysis, the selectivity for the formation of formaldehyde is less than 10% at conversions over 90%). Residence times have to be short and the cooling of the reaction products has to be rapid in order to lessen the decomposition of the formaldehyde which is not thermodynamically stable under the reaction conditions:
CH2Oxe2x86x92CO+H2
Various methods of carrying out this reaction have been proposed; thus, for example, DE-A-37 19 055 describes a process for preparing formaldehyde from methanol by dehydrogenation in the presence of a catalyst at elevated temperature. The reaction is carried out in the presence of a catalyst comprising at least one sodium compound at a temperature of from 300xc2x0 C. to 800xc2x0 C.
J. Sauer and G. Emig (Chem. Eng. Technol. 1995, 18, 284-291) were able to set free a catalytically active species, which they presume to be sodium, from a catalyst comprising NaAlO2 and LiAlO2 by means of a reducing gas mixture (87% N2+13% H2). This species was able to catalyze the dehydrogenation of methanol introduced at a downstream point in the same reactor, i.e. not coming into contact with the catalyst bed, to give formaldehyde. When using non-reducing gases, only a low catalyst activity was found.
According to J. Sauer and G. Emig and also results from more recent studies (see, for example, M. Bender et al., paper presented to the 30th annual meeting of German catalyst technologists, March 21-23, 1997), sodium atoms and NaO molecules were identified as species emitted into the gas phase and their catalytic activity for the dehydrogenation of methanol in the gas phase was described.
In the known processes, the starting material methanol is always diluted with nitrogen and/or nitrogen/hydrogen mixtures for the reaction.
Although good results are achieved with the known processes, there is nevertheless considerable room for improvement from a technical and economic point of view, particularly because the catalysts employed become exhausted or inactivated over time and the formaldehyde yields are still capable of improvement.
It has surprisingly been found that it is possible to increase the yield in the dehydrogenation of methanol by means o f an improved reaction procedure. This can be achieved by setting separate temperatures and possibly also residence times in the primary catalyst decomposition zone and the reaction section, particularly when the temperature level in the actual reaction section is set at a lower value than in the primary catalyst addition unit.
In this way, methanol conversions of more than 95% and high formaldehyde selectivities can be achieved and, surprisingly, non-reducing gases can also be used as carrier gas.
The invention accordingly provides a process for preparing formaldehyde from methanol by dehydrogenation in a reactor in the presence of a catalyst at a temperature in the range from 300 to 1000xc2x0 C., wherein the catalyst is generated spatially separately from the reactor and at a temperature above the dehydrogenation temperature.
Advantages of a lower reaction temperature are the lower energy and apparatus requirements for heating/cooling before/after the reaction, the low decomposition rate of the formaldehyde which is thermally unstable under the reaction conditions and the lower demands placed on the materials of construction .
The temperature difference is preferably at least 20xc2x0 C., particularly preferably from 40 to 250xc2x0 C.
When suitable primary catalysts are thermally treated in the primary catalyst decomposition zone and a reducing or even non-reducing gas such as molecular nitrogen is passed over them at a temperature which is not the same as the reaction temperature for the dehydrogenation, but is higher, one or more catalytically active species are released or generated and/or generated on them (secondary catalyst) and these species are able to catalyze the dehydrogenation of methanol. Such a fluid catalyst can be transported over considerable distances without suffering an appreciable loss of effectiveness in the dehydrogenation. This separate setting of the temperature allows, by matching to the respective conditions for catalyst liberation/vaporization or generation of a catalytically active species (secondary catalyst) on the one hand and for the reaction on the other hand, the possibility of, in particular, lowering the reaction temperature. This reduces the decomposition of the unstable formaldehyde as a result of subsequent reactions and increases the yield.
Preferred temperatures for generating the catalytically active species from the primary catalyst are from 300 to 1100xc2x0 C.; particular preference is given to temperatures of from 400 to 1000xc2x0 C.
Preferred temperatures for the dehydrogenation of the methanol are from 200 to 1000xc2x0 C.; particular preference is given to temperatures of from 300 to 980xc2x0 C.
Furthermore, the residence times in the dehydrogenation reactor and the vessel for primary catalyst addition or for generating the secondary catalyst can be set separately by dividing the carrier gas stream. A specific loading of the gas stream passed through the catalyst addition unit with the active species is achieved in this way.
Preferred residence times for generating the secondary catalyst are from 0.01 to 60 sec, particularly preferably from 0.05 to 3 sec, very particularly preferably from 0.05 to 1 sec. To dehydrogenate the methanol, preference is given to residence times in the reaction zone of from 0.005 to 30 sec, particularly preferably from 0.01 to 15 sec, very particularly preferably from 0.05 to 3 sec.
Replacement of the exhausted primary catalyst makes possible a continuous process for the non-oxidative dehydrogenation of methanol which gives improved methanol utilization or formaldehyde yields.
The carrier gas streams can consist of a reducing or a non-reducing gas, e.g. H2/CO mixtures or nitrogen, preferably the by-products of the dehydrogenation.